I'm facing the same problem (getting a good definition of SEM_VALUE_MAX)
for my posix_ipc extension. The patch presented here will get the
compiler to shut up on OpenSolaris, but not for the reason you think. At
least, that's the way I see it.

On OpenSolaris (2008.05 is what I'm testing with), _SEM_VALUE_MAX is
indeed #defined in sys/param.h, but it's inside this #ifdef on line 322:

#if (defined(_KERNEL) || defined(_KMEMUSER))

Since multiprocessing.c doesn't #define either of these, both
SEM_VALUE_MAX and _SEM_VALUE_MAX will remain undefined and so the patch
will always fall back to INT_MAX.

IMHO, given the choice between #defining _KERNEL or _KMEMUSER and
calling sysconf(_SC_SEM_VALUE_MAX), I feel safer doing the latter. 

I did a survey of all the operating systems to which I have access
(OpenSolaris 2008.05, OS X 10.5.5, RHEL?, Ubuntu 8.04, and FreeBSD 7).
OpenSolaris is the only one that doesn't #define SEM_VALUE_MAX, and on
all systems except for the possibly Red Hattish one,
sysconf(_SC_SEM_VALUE_MAX) returned the same value as the definition of
SEM_VALUE_MAX. I attached my code & results.